1. The present invention relates to the field of indexing mechanisms, most particularly those adapted for use with welding and drilling processes using beam energy, such as that provided by electron beams.
2. There are a great number of indexing mechanisms which are revealed by the prior art. Indexing mechanisms, as the name denotes, have as their function the sequential positioning of parts with respect to some fixed point. However, special problems arise when indexing is required to be undertaken in the harsh environment of an electron beam chamber. Not only does the presence of a high vacuum complicate the design, but the apparatus must be tolerable of the heat and shower of molten metal particles which are generated during the drilling process.
When specifically applied to the drilling of small diameter holes in the surface of a superalloy airfoil, the complexity of manipulation is increased. One reason for this is in the asymmetric nature of a gas turbine airfoil. Characteristically, the desired holes are oblique to the surface, which is often highly contoured in three dimensions. For practical purposes, in most electron beam drilling machines, the beam path is fixed within a small angle. Therefore, to put a row of holes in an airfoil, the part must be translated along the length of the row. To produce more than a single row on an airfoil, means must be further provided to translate the airfoil laterally with respect to the row length. To change the angle of any particular hole or part of a row, the part must be rotated with respect to the beam axis. If a multiplicity of airfoils are placed in a welding chamber at the same time, then further means must be provided for presenting each of the airfoils to the beam path. Thus, when a multiplicity of parts is sought to be simultaneously drilled with complex hole patterns, the cumulation of mechanical movements which are necessary can lead to systems which are unduly complex and therefore unreliable.
A further demand of electron beam hole drilling in airfoils is the accuracy which is desired. For example, a typical airfoil may require that holes of 0.45 to 1 mm diameter be spaced apart a distance of about 1.753.+-.0.254 mm, with the hole axes being aligned with better than 0.1 degree of the desired positions.
As might be expected, there are a great number of known devices for indexing parts in rotary manner. U.S. Pat. No. 3,532,009, Hogan et al., shows a typical apparatus where rotational motion to the nominal position is accomplished by an indexing pawl, and thereafter a final, exact position is accomplished by a second independently operated positioning plunger. U.S. Pat. No. 2,985,038, Tandler et al., shows a relatively complex peg and pawl positioning system. Mead, in U.S. Pat No. 2,968,973, shows a chain-motivated rotary table where the nominal position is achieved by means of a spring-loaded pin; the pin is disengageable by means of a linkage activated by the same prime mover piston as activates the chain. As reference to the foregoing patents will indicate, the prior art mechanisms for rotational indexing are comprised of a great number of elements. For use in a vacuum chamber necessary for electron beam drilling, it is desirable that the number of elements be reduced, that the number of actuators or actuator rods be reduced, and that the prime movers be located external to the chamber.